This invention pertains to devices (termed xe2x80x9cchucksxe2x80x9d) that hold a substrate, such as a semiconductor wafer coated with a xe2x80x9cresist,xe2x80x9d while a process such as microlithography is being conducted on the substrate. Microlithography concerns the transfer of a pattern, usually defined by a reticle, mask, or the like, onto the resist-coated surface of the substrate using an energy beam. Microlithography is a key technology used in the manufacture of microelectronic devices such as integrated circuits, displays, and the like. More specifically, the invention pertains to electrostatic chucks that hold the substrate by electrostatic attraction (force) between the chuck and the substrate. Even more specifically, the invention pertains to electrostatic chucks configured to dissipate heat from the substrate to the chuck using a heat-transfer gas.
As used herein, a xe2x80x9cwaferxe2x80x9d encompasses any of various types of substrates that can be held on a chuck, including semiconductor wafers, glass or ceramic plates, or any of various other suitable substrates.
In various microlithographic exposure processes involving the transfer of a pattern to a wafer, the upstream-facing surface (xe2x80x9cprocess surface,xe2x80x9d usually coated with a suitable resist) of the wafer is irradiated by an energy beam such as a beam of electromagnetic radiation or a charged particle beam. Absorption of the beam energy by the wafer causes wafer heating. The temperature rise usually is accompanied by thermal expansion of the wafer. Normally, wafer heat is conducted and dissipated to the chuck to which the wafer is mounted. However, any particulate debris situated between the downstream-facing surface of the wafer and the wafer-mounting surface of the chuck can cause deformation of the wafer. This deformation decreases the accuracy and precision of exposures on the wafer.
To solve these problems associated with particulate contamination, multiple channels conventionally are machined into the wafer-mounting surface of the chuck, thereby forming respective xe2x80x9cgapsxe2x80x9d or spaces between the wafer and the chuck. To facilitate thermal conduction in the regions of the gaps, a heat-transfer gas conventionally is circulated through the channels while the wafer is mounted to the chuck. A typical heat-transfer gas is helium (He) gas. Circulation of the heat-transfer gas through the channels in this manner reduces thermal expansion of the wafer because the gas (which contacts the under-surface of the wafer in the gap regions) conducts heat from the wafer to the gas and from the gas to the chuck. Areas of the wafer-mounting surface in which channels are not formed contact the wafer directly and thus directly conduct heat from the wafer to the chuck.
Parameters influencing the amount and rate of thermal transfer to the chuck by the heat-transfer gas include the thermal conductivity of the gas, the gas pressure, the gas-flow rate, and transverse dimensions of the respective gaps. For instance, if the mean free path of molecules of the heat-transfer gas is longer than a minimum transverse dimension of a gap, then at relatively low gas pressure the thermal conductivity of the gas increases nearly proportionally with increases in gas pressure. However, if the mean free path is shorter than a minimum transverse dimension of a gap, then the thermal conductivity of the gas is essentially independent of gas pressure.
Especially for charged-particle-beam (CPB) microlithography, the chuck is situated in a vacuum chamber evacuated to a suitable vacuum level using a vacuum pump. Under such conditions, it is important that leakage of the heat-transfer gas from the chuck into the vacuum chamber be as low as possible. Excessive leakage of heat-transfer gas can compromise the performance of the vacuum pump to an extent such that the required vacuum level in the vacuum chamber is not achievable or maintainable. In such an event, adverse consequences may result such as reduced accuracy or precision of pattern transfer, or damage to the microlithography apparatus.
One conventional method for controlling leakage of heat-transfer gas involves providing a ring-shaped protrusion around the perimeter of the wafer-mounting surface of the chuck. Just inward (in the radial direction) of the protrusions are gas-exhaust ports for aspirating residual heat-transfer gas in the channels. Other methods include monitoring of the pressure of the heat-transfer gas and controlling the flow rate of the heat-transfer gas in gas inlets to the channels. These methods are generally acceptable for maintaining the pressure of the heat-transfer gas as constantly as possible during wafer exposure, thereby achieving effective transfer of heat away from the wafer.
In some microlithography apparatus, however, chucks are used that include a mechanism for moving the wafer vertically relative to the wafer-mounting surface to facilitate effective adhesion of the wafer to and detachment of the wafer from the chuck. Such mechanisms work in concert with a robot arm used for transporting the wafer to and from the chuck, and eliminate the need for the robot arm to move vertically relative to the chuck. Conventional mechanisms for vertical movement of the wafer relative to the chuck extend through feed-through holes in the surface of the chuck. The feed-through holes also facilitate pressure equalization allowing the wafer to be lifted off the chuck surface. Unfortunately, the feed-through holes provide conduits for escape of heat-transfer gas from the channels to the vacuum chamber, thereby causing undesired changes in pressure of the heat-transfer gas in the gaps and hindering wafer processing.
In view of the shortcomings of conventional devices and methods as summarized above, an object of the invention is to provide electrostatic chucks that can suppress leakage of heat-transfer gas more effectively from the gap between the wafer and the chuck, thereby more reliably maintaining the pressure of the heat-transfer gas in the gap.
To such end, and according to a first aspect of the invention, electrostatic chucks are provided for holding a substrate for processing. An embodiment of such a chuck comprises a chuck body. The chuck body defines a wafer-mounting surface configured to contact an under-surface of a substrate whenever the substrate is electrostatically mounted to the chuck. The chuck body also defines a gap between the under-surface of the substrate and the chuck whenever the substrate is mounted to the wafer-mounting surface, and multiple feed-through holes extending through a thickness dimension of the chuck body. The chuck body also defines a gas-inlet port situated and configured to conduct a heat-transfer gas to the gap so as to allow the heat-transfer gas to contact the under-surface of the substrate whenever the substrate is mounted to the wafer-mounting surface. The chuck body also defines a gas-exhaust port situated and configured to conduct the heat-transfer gas away from the gap. The chuck body also defines a respective first protrusion surrounding each feed-through hole, wherein the first protrusions are configured to separate the respective feed-through hole from the gap. The chuck embodiment also includes a respective lift pin situated in each feed-through hole. The lift pins are configured to extend from the chuck body across the gap to the under-surface of the substrate. The lift pins are actuatable relative to the chuck body so as to contact the under-surface of the substrate and cause lifting of the substrate relative to the wafer-mounting surface.
The chuck summarized above can include a gas-exhaust pump connected to the gas-exhaust port and configured to remove heat-transfer gas from the gap, and a gas supply connected to the gas-inlet port and configured to supply the heat-transfer gas to the gap.
The chuck body can further define a first peripheral protrusion extending circumferentially around the chuck body, wherein the peripheral protrusion has a height, relative to the chuck body, equal to a height of the respective first protrusions, around the feed-through holes, relative to the chuck body. In this configuration, the chuck body can define multiple gas-exhaust ports situated radially inwardly of the first peripheral protrusion.
The chuck body can further define a second peripheral protrusion extending circumferentially around the chuck body radially inwardly of the first peripheral protrusion, wherein the second peripheral protrusion has a height, relative to the chuck body, equal to the height of the first peripheral protrusion. In this configuration, the chuck body can define multiple gas-exhaust ports situated radially between the first and second peripheral protrusions.
The chuck body can further define, at each feed-through hole, a respective second protrusion surrounding the respective first protrusion, wherein the second protrusion also is configured to separate the respective feed-through hole from the gap. In this configuration, at each feed-through hole, the respective second protrusions desirably have a height, relative to the chuck body, equal to a height of the respective first protrusions. The chuck body can further define, at each feed-through hole, a respective gas-exhaust port situated between the first and second protrusions surrounding the respective feed-through hole. The chuck body can further define, at each feed-through hole, multiple respective gas-exhaust ports situated between the first and second protrusions surrounding the respective feed-through hole.
According to another aspect of the invention, microlithography apparatus are provided that comprise a chuck according to any of the embodiments summarized above.
With any of the various configurations according to the invention, the amount of heat-transfer gas escaping through the feed-through holes around the lift pins is substantially decreased compared to conventional chucks. This allows the pressure of the heat-transfer gas in the gap to be more accurately and reliably maintained during use of the chucks.
In the configurations in which the chuck body defines multiple protrusions surrounding each feed-through hole, especially with one or more gas-exhaust ports situated between the protrusions, leakage of heat-transfer gas from the gap is even further reduced.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.